Getting mouse position relative to movie clip
If you run this script in Blender's text editor, you can press L with your mouse in the preview window to get the position of the mouse in pixels relative to the bottom left corner of the video clip.

import bpy

bl_info = {"name": "Mouse Pos", "category": "Sequencer"}

class GetMousePos(bpy.types.Operator):
    bl_idname = "mouse_pos.get_mouse_pos"
    bl_label = "Get Mouse Pos"

    def invoke(self, context, event):
        # Size of the preview region
        width = context.region.width
        height = context.region.height

        fac = 1.0

        prs = bpy.context.space_data.proxy_render_size
        res_perc = bpy.context.scene.render.resolution_percentage

        if prs == 'SCENE':
            fac = res_perc / 100

        rv1 = context.region.view2d.region_to_view(0, 0)
        rv2 = context.region.view2d.region_to_view(width, height)

        preview_zoom = (width / (rv2[0] - rv1[0]))

        # Resolution of the scene
        res_x = context.scene.render.resolution_x
        res_y = context.scene.render.resolution_y

        # Distance of video preview from bottom left corner
        offset_x = -int((rv1[0] + ((res_x * fac) / 2)) * preview_zoom)
        offset_y = -int((rv1[1] + ((res_y * fac) / 2)) * preview_zoom)

        print(event.mouse_region_x - offset_x, event.mouse_region_y - offset_y)

        return {'FINISHED'}

def register():
    bpy.utils.register_class(GetMousePos)

    keyconfig = bpy.context.window_manager.keyconfigs['Blender Addon']
    km = keyconfig.keymaps["SequencerPreview"]

    kmi = km.keymap_items.new("mouse_pos.get_mouse_pos", 'L', 'PRESS')

def unregister():
    bpy.utils.unregister_class(GetMousePos)

    keyconfig = bpy.context.window_manager.keyconfigs['Blender Addon']
    km = keyconfig.keymaps["SequencerPreview"]
    for kmi in km.keymap_items:
        if kmi.idname == "mouse_pos.get_mouse_pos":
            km.keymap_items.remove(kmi)

    bpy.utils.unregister_module(__name__)

register()